Simplified Explanation of Digitizing Tablet
FIG. 1 illustrates a computer 4 and a digitizing tablet 3, which provides input to the computer. (The digitizing tablet need not be a separate element, as shown in FIG. 1, but can take the form of a transparent overlay of the display 2. This overlay configuration is frequently used in pen-based portable computers.) The tablet-and-stylus combination provides input to the computer. A simplified explanation of the operation of the tablet will be given.
FIG. 2 shows a digitizing tablet 3 and its associated stylus 6. In the tablet, there are four current-to-voltage amplifiers (labeled I-to-V), one at each corner. The stylus 6, when activated, produces a signal 9, as indicated in FIG. 3. This signal induces currents I1, I2, I3, and I4, which are detected by the I-to-V amplifiers.
The I-to-V amplifiers each produce a voltage (V) indicative of the size of its respective current (I). Processing circuitry, not shown, but known in the art, receives the voltage signals, and computes the position of the stylus 6.
The currents I1-I4 are induced because the stylus 6 acts as one plate of a capacitor. The digitizing tablet supplies the other plate: it bears a resistive surface, or grid, of a material such as indium tin oxide, which acts as the other plate.
As FIG. 4 indicates, when negative charge is applied to the tip 5 of the stylus 6, a positive charge is induced on the surface of the tablet 3. Currents I1-I4 supply this positive charge. Conversely, as in FIG. 5, a positive charge on the tip 5 of the stylus 6 induces a negative charge on the tablet. The currents I1-I4 supply this negative charge.
As FIG. 6 shows, each current can be viewed as following a direct path from the stylus to one corner of the tablet. The following reasoning allows stylus position to be inferred from the voltage signals:
1. The length of each path determines the resistance of the path. PA1 2. The resistance determines the size of the current. PA1 3. The size of the current determines the voltage produced by the amplifier. PA1 4. Each voltage indicates its respective path length. A highly simplified example will illustrate the above-described inference.
Assume that the stylus in FIG. 6 represents a voltage source applied to the tablet. The paths have the relative lengths shown in FIG. 6, and also listed in the following Table.
TABLE RELATIVE RELATIVE RELATIVE RELATIVE I-to-V PATH LENGTH RESISTANCE CURRENT VOLTAGE AA 0.66 0.66 1.52 1.52 BB 1.00 1.00 1.00 1.00 CC 0.93 0.93 1.08 1.08 DD 0.55 0.55 1.82 1.82
The resistance of the path depends on the length, so that the relative resistances are in proportion to the relative path lengths, as indicated in the Table's columns RELATIVE LENGTH and RELATIVE RESISTANCE.
Current equals voltage divided by resistance, V/R, so that the relative currents will be inversely proportional to the relative path lengths, as indicated in the Table's column RELATIVE CURRENT.
The I-to-V amplifiers in FIG. 2 each produce a voltage which is proportional to its respective current. Thus, the voltage outputs of the I-to-V amplifiers have the relative magnitudes indicated in the rightmost column of the Table.
The output voltage signals can be used to compute position of the stylus.
Signature Recognition
The system described above can be used to recognize a user's signature. FIG. 7 shows the stylus 6 writing a signature. The position of the stylus 6 is detected periodically, such as every 1/100 second, or other suitable interval. Data points, indicated by the crosses in FIG. 8, are derived at these intervals.
For each data point, the x- and y-position are known, together with the real-time occurrence of the data point. Consequently, velocities, accelerations, and other parameters, in addition to the x- and y-positions, can be derived.
Each person's signature is represented by a unique collection of these parameters. Thus, these parameters, like fingerprints, can be used to identify a person.
Tether Not Required
Some styluses are not tethered to the computer by the power cord P shown in FIG. 1; instead, they are battery-powered.